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Paraphyletic)
In phylogenetics, a grouping of organisms is said to be paraphyletic (Greek para = near and phyle = race) if the group contains the most recent common ancestor of its members, but the group does not include all the descendants of this common ancestor.
Reptiles are a paraphyletic group. The group can be made monophyletic by including the birds (aves).
Groups that include all the descendants of the most recent common ancestor are commonly termed monophyletic or holophyletic. The former is more common, but sometimes paraphyletic groups are also considered monophyletic, in which case the latter is used. A paraphyletic group can be fixed by expanding it, and including other clades. A class which does not contain the most recent common ancestor of its members is called polyphyletic.
Many of the older classifications contain paraphyletic groups, especially the traditional 2-6 kingdom systems and the classic division of the vertebrates. For example, the class Reptilia as traditionally defined is paraphyletic because that class does not include birds (class Aves), which are descended from reptiles. Paraphyletic groups are often erected on the basis of plesiomorphies (ancestral similarities) instead of upon apomorphies (derived similarities).
In most cladistics-based schools of taxonomy, the existence of paraphyletic groups in a classification is regarded as an error. Some groups in currently-accepted taxonomies may later turn out to be paraphyletic, in which case the classifications may be revised to eliminate them. Some, however, feel that having paraphyletic groups is an acceptable sacrifice if it makes the taxonomy more understandable. Others argue that paraphyletic groups are necessary to have a comprehensive classification including extinct groups, since each species, genus, and so forth necessarily originates from part of another. It has been suggested that paraphyletic groups should be allowed, but clearly marked as such, for instance in the form Reptilia*. The term "evolutionary grade" is sometimes used for such groups.
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